This invention relates to a frame offset setting arrangement and a frame offset setting method for use in a mobile communication system and, in particular, to a frame offset setting arrangement and a frame offset setting method for reducing a processing delay resulting from concentration of speech packets on a base station control unit at a same time instant or slot.
In a CDMA (Code Division Multiple Access) system defined by a communication standard IS-95 recommended by the U.S. TIA (Telecommunications Industry Association), a frame offset value is informed from a base station (more specifically, a base station control unit of the base station) to a mobile unit upon call connection. Assuming that each frame has a unit time of 20 ms, the frame offset value represents one of 0-th through 15-th positions determined by dividing the unit time of 20 ms into 16 equal parts. Typically, the frame offset value indicates one of integers 0 through 15 to represent the 0-th through the 15-th positions, respectively. The 0-th through the 15-th positions are offset by 1.25 ms from one another. Informed of the frame offset value, the mobile unit shifts a transmission timing of a speech packet from a leading edge of each frame by (the frame offset valuexc3x971.25 ms). The frame offset value is determined by the base station control unit so that the speech packets are not temporarily concentrated on a transmission path between a base station radio unit of the base station and the base station control unit.
It is assumed here that speech packets are simultaneously supplied from all mobile units connected to a particular base station. In this event, a processing delay is caused to occur when the speech packets are inserted into the transmission path. In other words, the processing delay occurs when the speech packets from the mobile units are made to pass through the transmission path between the base station radio unit and the base station control unit. In this case, the processing delay can be reduced if the transmission timings of the speech packets are shifted by the mobile units which are informed of the frame offset values different from each other as described above. In this manner, it is possible to reduce the processing delay upon insertion of the speech packets from the base station radio unit to the transmission path.
In the base station control unit of the above-mentioned prior art, the speech packets from a plurality of base station radio units are converted from QCELP (Qualcomm Codebook Excited Linear Production) as a radio zone modulation system into PCM as a network modulation system. In this event, a processing delay may possibly be produced because a number of speech packets to be simultaneously converted are received by the base station control unit depending upon the distribution of the frame offset values determined by the base stations.
Specifically, in the above-mentioned prior art, it is assumed that a particular mobile unit moves from an area of a particular base station radio unit to a different area of a different base station radio unit by handover (changing a communication channel). In this event, a particular frame offset value before the handover is used without being changed. Therefore, in the different base station radio unit in the different area, the particular frame offset value for the particular mobile unit subjected to handover may coincide with the same frame offset value used for other mobile units within the different areas so that this frame offset value is concentratedly used. In this event, the speech packets to be converted are concentrated. This results in occurrence of the processing delay, speech omission, speech distortion, and data omission.
Generally, the frame offset value are not concentratedly used because they are determined at random. However, in case of the above-mentioned handover or if calls of a long speech period concentrate on a specific frame offset value, concentrated use will occur.
Now, description will be made about an example of the processing delay resulting from the concentrated use of the frame offset values with reference to FIGS. 1 and 2. At first referring to FIG. 1, it is assumed that a plurality of (xe2x80x9c3xe2x80x9d in the illustrated example) mobile units transmit signals (or speech packets) a, b, and c to a single base station radio unit 200 at the timings specified by the same frame offset value xe2x80x9c0xe2x80x9d. Inasmuch as the signals (or the speech packets) a, b, and c are transmitted by the same frame offset value xe2x80x9c0xe2x80x9d, the base station radio unit 200 transmits to a base station control unit 100 an exchanged frame including the signals (or the speech packets) a, b, and c in the order of the signals (or the speech packets) a, b, and c. Thus, a delay is produced for each of the signals b and c. In this case, such frames are accumulated in a buffer provided in a digital transmission interface DTI 2 of the base station radio unit 200 to cause a signal delay.
Next referring to FIG. 2, it is assumed that a traffic channel controller TCHC 1 in the base station radio unit 100 extracts a speech signal of a highest quality among those transmitted from base station radio units 200-1 through 200-3. In this event, a delay is produced if the same offset value is used by a large number of mobile units. Specifically, no problem will arise in each of the base station radio units 200-2 and 200-3 because each of the base station radio units 200-2 and 200-3 does not receive any signals or speech packets which are produced by different mobile units on the basis of the same offset value. On the other hand, the base station radio unit 200-1 receives signals (or speech packets) a and b which are produced by different mobile units on the basis of the same offset value. Therefore, the delay is produced for the signal or packet b.
Japanese Unexamined Patent Publication (JP-A) 6-197097 discloses a technique of giving a transmission timing offset to a signal transmitted from a base station so that each communication can be identified although a same spread code is used. Japanese Unexamined Patent Publication (JP-A) 9-261763 discloses a mobile communication system in which a pilot signal transmitted from each base station is provided with a timing offset specific to the base station so that the mobile unit can reliably identify each base station. However, in these publications, no disclosures is made about the defect due to the concentrated use of the frame offset values. Thus, use of the techniques described in these publications does not remove the above-mentioned disadvantage in the prior art.
It is an object of this invention to provide a frame offset setting arrangement and a frame offset setting method which are capable of reducing speech omission, speech distortion, and data omission resulting from a processing delay when speech packet are transmitted from a base station radio unit through a base station control unit to an exchange.
A frame offset setting arrangement to which this invention is applicable is arranged in a base station (100, 200-i) of a code division multiple access system for setting frame offset values in mobile units (300-1, 300-2, 300-3) by informing the frame offset values to the mobile units. The frame offset values represent amounts of offsetting, from a leading edge of each frame, transmission timings of speech packets when the mobile unit transmit the speech packets to the base station, respectively.
According to this invention, the frame offset setting arrangement comprises: concentration detecting means (110, 111) for detecting concentration in distribution of the frame offset values set in the mobile units; and leveling means (112-121) for leveling, when the concentration detecting means detects the concentration, the frame offset values for the mobile units which are currently on communication.
A frame offset setting method to which this invention is applicable is carried out in a base station (100, 200-i) of a code division multiple access system for setting frame offset values in mobile units (300-1, 300-2, 300-3) by informing the frame offset values to the mobile units. The frame offset values represent amounts of offsetting, from a leading edge of each frame, transmission timings of speech packets when the mobile units transmit the speech packets to the base station, respectively.
According to this invention, the frame offset setting method comprises: a concentration detecting step (110, 111) of detecting concentration in distribution of the frame offset values set in the mobile units; and a leveling step (112-121) of leveling, when the concentration detecting step detects the concentration, the frame offset values for the mobile units which are currently on communication.
In this invention, the base station monitors and detects the concentration in distribution of the frame offset values. If the concentration is detected, the frame offset values are reset to use different frame offset values. More specifically, the frame offset values currently used are monitored and, if the concentration is detected, calls currently on communication at a current base station is selected to forcedly start handover and reset the frame offset values. In this manner, the concentration in distribution of the frame offset values is cancelled.